The present invention relates to solid state circuitry for switching relatively high currents in response to a control signal.
In numerous electrical systems, the conventional, electro-mechanical relay still provides useful characteristics which are not readily obtainable in solid state circuitry. For example, conventional relays have the advantage of providing complete electrical isolation between the control signal, that is the signal applied to the relay coil, and the external circuit controlled by the opening and closing of the relay contacts. In contrast, most solid state switching devices and circuits exhibit electrical coupling of the control and controlled circuits such that it is necessary to resort to the older, more conventional switching devices, namely relays, when it is desirable to maintain isolation between the control signal and the switched circuitry.
Furthermore, relays are typically capable of handling substantial current flow between the contacts, whereas solid state circuits capable of achieving the same current switching capabilities generally require expensive and sophisticated devices.
Other situations arise where solid state circuitry might be employed; however, difficulties arise in adapting the solid state switching components to a system which has been designed for use with relays. In such case, the existing system may be incapable of operating satisfactorily without providing the same characteristic switching parameters including isolation between the input control signal and the output or controlled circuit as is available from a relay.
Notwithstanding the continued utility of relay switching devices, it is recognized that relays exhibit certain disadvantages in their switching characteristics. Among these disadvantages are contact bounce, unpredictable switching times, relatively slow switching times compared to solid state devices, arcing between the contacts during opening, especially in the presence of highly inductive loads, and limited standoff voltage capability. Although various types of remedial circuitry has been devised for ameliorating the effects of these disadvantageous characteristics, the relay, in general is incapable of matching the switching speeds and predictability of turn on and turn off times exhibited by solid state switching devices.
In view of the foregoing, numerous attempts have heretofore been made to adapt solid state circuits for use as replacements or substitutes for conventional relays. For example, a single switching transistor may be combined with various current directing diode networks connected across the switched output terminals of the transistor in an effort to simulate the bidirectional current flow capabilities of a pair of relay contacts. However, such circuits have not always been found satisfactory. In another example, solid state triac devices have been employed as voltage controlled switches; however, these also have been found lacking in certain characteristics, such as the undesirably large voltage drop across the switched terminals when the device is driven to a conducting "on" condition.